I am not very scared about the future movies. I trust Chris Columbus very, very much. I think he did not talk about the prophecy because he wanted to keep some good stuff for the future ones… There was no actions about the prophecy in the first book but talking and stories and such. If the movie was overloaded with information that aren't necessary for the moment, the future movies would have been incomplete, and films are usually made for everybody, and even people who did not see the first titles should understand. Motion picture and literature are completely different industries.
I think Clarisse will show up when she will be needed in the adventure, they won't pay an actress for two scenes anyway, and they don't want to mix up the audience, because they are not all angry fans! Anyway, I don't know if this seems clear, because I'm not really good in explaining what I think… I think that the Lightning Thief, the book, and the movie, are two different things, and both are fantastic. And I was stupidly happy Clarisse and Thalia didn't show up, because I had some tiny innocent hopes about the crew doing public castings. Silly thoughts, since I live in Quebec! (who cares, really)

Percy Jackson and Chris Colombus are best friend now. Please, don't insult him in front of me. He's my idol since Harry Potter 1. Percy Jackson's fans are the coolest fans of the world!

but i agree with you the movie was great.. and im not sure but didnt it say BASED on the best selling series.. so i dont think it was actually made to be exactly like the book but then agian i could be remembering wrong : ) but when I saw the movie I was really impressed aside from all the parts they changed and whenever anyone says ''idk if i should see that'' I tell them that it was an awsome movie cuz it was XD

I think a reason why ppl are complaining is because it was directed by Chris Columbus and were looking forward to how great he would make it because of his previous work on HP 1 AND HP 2

The fact that they left out the prophecy was disappointing. It felt like they weren't thinking about potential sequels at all, weren't trying to build a series, but just wanted to make a stand-alone film. Having said that I think they can easily slot the prophecy back into the Sea of Monsters, either as something Chiron has already known about but didn't want to reveal (which is pretty much what happened in the books anyway) or as a prophecy that was just made.

What I'm more worried about is the lack of Kronos and the Annabeth/Luke/Thalia back story. Not so much the Thalia part actually, I think she too can be easily slotted in to a sequel (along with Clarisse) but the Annabeth/Luke connection which actually ends up being important at the end … not sure how they can fix that given that they didn't show any kind of connection b/w them at all, and Annabeth fought him at the end without any reservations.

Not mentioning Kronos as the puppet pulling the strings is a pretty huge omission to. Like a Philosopher's Stone film with no mention of Voldemort – insane. Not entirely sure how they can fix that …

Those are the only two things I'm really worried about though, I think everything else will be okay. Let's just hope we actually get those sequels! (And hope that, whether they bring back the same screenwriter or not) that they're a bit more faithful to the books from here on!